1. Field of the Invention
This invention relates in general to digital communications implemented using a frequency hopping spread spectrum technology. In particular, the invention relates to a technique for reliably transmitting long messages spanning multiple packets over a noisy frequency-hopped communications channel.
2. Background Art
As the prevalence and diversity of electronic devices incorporating digital communications techniques increases, it becomes important to implement versatile communications protocols that are well-suited to varied message contents and highly tolerant of noisy communications channels.
Frequency hopping spread spectrum (“FHSS”) is an increasingly popular technique for implementing a digital communication link. FHSS involves the rapid alteration of a transmitted signal's carrier frequency in a known pattern, called the hop sequence. One advantage of a FHSS implementation is that interference sources occupying a portion of the communications channel do not completely disrupt the communication link; rather, only communications on hop sequence channels coinciding with an interference source are disrupted. Furthermore, in some circumstances, the carrier frequencies in the hop sequence can be dynamically allocated to eliminate frequencies at which interference is present, thus dynamically preserving a reliable communications link.
However, for some types of FHSS communications it is desirable to utilize a fixed hopping pattern. One example of this is for broadcast messaging to a plurality of remote receivers, such as that often employed by a cordless telephone system with multiple handsets and a single base unit. While the handsets are not “in use”, they must be able to receive general messaging from the base unit to set up incoming calls, receive caller ID information, or perform other such functions. However, implementation of dynamic channel allocation with multiple receiving radios is often difficult because each channel substitution must be received and processed by every receiver without error. Also, handsets completing a call and returning to a standby state must become aware of changes in the broadcast hop sequence. Therefore, it is desirable to implement a fixed hop sequence for broadcast communications.
One substantial complication inherent to communications over a fixed hop sequence arises during the broadcast of long messages. Long messages are deemed those which contain an amount of data greater than that which can be transmitted during a single carrier frequency channel in the hop sequence. Thus, such messages are commonly broken up and transmitted via a plurality of data subpackets on different channels in the hop sequence. The message is then reconstructed and processed by the receiving device after all of the packets have been received. However, such messages are highly prone to errors inasmuch as an error in any one of the multiple transmitted packets will corrupt the entire message once it is reassembled by a receiving device.
The susceptibility of long messages to errors is further exacerbated by the use of a fixed hopping pattern. In circumstances when a large number of the channels in the hopping pattern are corrupted by interference, the hopping pattern may not even contain a number of consecutive “interference-free” channels sufficient to transmit an entire long message. Broadcast of long messages then becomes virtually impossible over such a highly noisy communications channel. Therefore, it is an object of this invention to enable the reliable communication of long messages over a noisy communications channel.
Even when channel noise does not completely prevent long message broadcast, the susceptibility of prior art fixed hopping pattern techniques to interference typically requires that a form of communications acknowledgement be implemented. An acknowledgement function would have the receiving handset(s) send back to the base some indication as to whether or not a long message was received without error. However, several disadvantages are inherent in the implementation of such an acknowledged communications protocol. The broadcasting device incurs added complexity to be able to also monitor one or more additional channels over which it would receive acknowledgement messaging from downlink recipients. Also, the downlink recipient devices over time consume substantial power by powering up their respective transmitters just to transmit an acknowledgement. In applications such as cordless telephones, where the broadcast recipients attempt to minimize power consumption due to limited battery capacity, the power consumption required to regularly transmit acknowledgements can comprise a substantial disadvantage. Therefore according to another aspect of this invention, a reliable long message broadcast protocol is provided that is also unacknowledged.
These and other objects of the present invention will become apparent to those of ordinary skill in the art in light of the present specifications, drawings and claims.